The present invention relates to a method for isolating and purifying antibiotics. Antibiotics are a group of medicinal substances that are suitable for fighting infectious diseases. Antibiotics are usually produced by a microorganism and are capable of inhibiting or killing the growth of other microorganisms. Included in the antibiotics group are penicillins, cephalosporins, tetracyclines, aminoglucoside antibiotics, nucleoside antibiotics, macrolide antibiotics, ansamycines, peptide antibiotics and antibiotics having unique structures. By far, most antibiotics are formed in fermentation processes under sterile conditions by microorganisms which are cultivated in aerated nutrient solutions. When a culture has reached a sufficiently high content of antibiotics formed by the microorganisms, it is harvested, typically by drying, particularly spray drying, the entire culture. Alternatively, the culture may be harvested by filtration and subsequent processing of the antibiotic containing culture solution or the antibiotic containing mycelium by multiple-stage isolation and purification processes, wherein in a first stage extraction takes place and in a second stage precipitation of the extracted antibiotics occurs. It is also possible to directly precipitate the antibiotics in the first process stage from the filtered out antibiotic containing culture solution. The intermediate products obtained by precipitation must be purified, which is done, in particular, by repeated recrystallization and reprecipitation. Since for various reasons many antibiotics quickly decompose when in the dissolved state, their isolation and purification must be effected as quickly as possible. Moreover, care must be taken when isolating and purifying antibiotics to be sure that they are not overheated as most of these compounds decompose at higher temperatures.
In the known processes used for isolating and purifying antibiotics, antibiotic solutions are obtained in the individual process stages, which contain organic solvents in addition to water. Not only the antibiotics but also the organic solvents must be removed from these solutions before they can be discharged into the waste water of the manufacturing faciity. Moreover, the multiple stages of the isolating and purifying processes result in yield losses and operating malfunctions.
U.S. Pat. No. 3,969,196 to Zosel discloses a process for separating mixtures of liquid or solid substances in which the mixture is treated with a gas that is under supercritical conditions with respect to temperature and pressure, wherein the temperature range of the gas is up to more than 100.degree. C. above its critical temperature. After separating the charged supercritical gas phase, the compounds contained therein are recovered by expansion or by an increase in the temperature of the gas. However, this reference would not lead one skilled in the art to arrive at the process of the present invention because it does not provide any indication that antibiotics can be extracted, as it relates to the extraction of simpler organic compounds. Moreover, it does not teach the possibility of conducting a separation with an extraction agent whose critical temperature lies in the range from 0.degree. to 160.degree. C. at a temperature which may be below the critical temperature, that is, from 0.degree. to 40.degree. C., and at a pressure which lies between the critical pressure and the ten-fold value of the critical pressure of the extraction agent. Nor does it teach the precipitation of the antibiotics at a temperature from 0.degree. to 40.degree. C. Furthermore, this reference does teach that separation processes involving organics, even if confined to gases under supercritical conditions, are not predictable as they depend on the specific constitution of the particular compound. Testing a small sample is recommended in order to ascertain the probability of success. Surprisingly, we have found that antibiotics can be separated from culture solutions as a general practice using gases under pressure which are not necessarily in the supercritical state.